This invention relates to a process for decomposing ozone by contacting an ozone-containing gas with a catalyst comprising an AMS-1B crystalline borosilicate-based catalyst.
Because of its strong oxidative properties, ozone is used for disinfecting and removing undesirable organic material such as bacteria from waste water, sewage, drinking water, and industrial stack gases. Ozone is generated for use in many chemical processes which require the presence of a strong oxidant. The uses of ozone are described in Kirk-Othmer's, Encyclopedia of Chemical Technology, Third Edition, Vol. 16, pp. 683-712, incorporated herein by reference. Unfortunately, ozone is biologically toxic and remains chemically active, even in concentrations of less than 1 ppm. When an ozone-containing gas is released into the atmosphere, the resulting photochemical reaction generates oxidant, thereby causing secondary environmental pollution. The control of ozone is a particular problem in certain situations where ozone is generated or maintained for a long period in an enclosed area (e.g., when it is generated during the operation of equipment such as electronic photocopying machines or found in airplanes flying at high altitudes).
Ozone decomposition processes use different catalytic compositions. Examples of such compositions include an activated carbon-zeolite mixture (U.S. Pat. No. 4,259,299), deposited metallic silver and a composite of zirconia and oxides of manganese (U.S. Pat. No. 4,261,863), and a platinum group metal or compound and a nonprecious Group VIII metal oxide or aluminate (U.S. Pat. No. 4,405,507). A number of catalysts for the decomposition of ozone at room temperature were evaluated by Ellis et al. (Atmospheric Environment, Vol. 6, Pergamon Press 1972, pp. 707-714). Among these were charcoal, metals and metal oxides, zeolite catalyst and silica-alumina catalyst. Charcoal and nickel oxide were found to be the best catalysts tested while the zeolite and silica-alumina catalysts were among the least efficient in decomposing ozone. After 15 minutes of contact with a zeolite or silica-alumina catalyst, an air stream containing ozone had only about 0-10% of the ozone removed. In U.S. Pat. No. 4,183,728, aluminosilicates were used to adsorb nitrogen oxides in a gas mixture containing ozone. The reference specifically teaches that the ozone passes over the aluminosilicate without any decomposition. When charcoal is used to decompose ozone, the charcoal mass is consumed during the process. Also, possible dust from a charcoal catalyst would prove inconvenient in the situations involving enclosed areas such as airplanes. Catalytically active metals are susceptible to "poisoning" by gases such as carbon monoxide, ammonia, sulfur dioxide or hydrogen disulfide.
Zeolitic materials, both natural and synthetic, are many times known to have catalytic capabilities. Zeolitic materials typically are ordered porous crystalline aluminosilicates having a definite structure with cavities interconnected by channels. The cavities and channels throughout the crystalline material generally are uniform in size, allowing selective separation of hydrocarbons. Consequently, in many instances these materials are known in the art as "molecular sieves" and are used, in addition to selective adsorptive processes, for certain catalytic properties. The catalytic properties of these materials are affected to some extent by the size of the molecules which selectively penetrate the crystal structure, presumably to contact active catalytic sites within the ordered structure of these materials.
Boron is not considered a replacement for aluminum or silicon in a zeolitic composition. However, recently a new crystalline borosilicate molecular sieve AMS-1B with distinctive properties was disclosed in U.S. Pat. Nos. 4,268,420 and 4,269,813, incorporated by reference herein. According to these patents, AMS-1B can be synthesized by crystallizing a source of an oxide of silicon, an oxide of boron, an oxide of sodium, and an organic template compound such as a tetra-n-propyl ammonium salt. The process of this invention uses AMS-1B crystalline borosilicate molecular sieve.
It is a general object of this invention to provide an effective, convenient method of treating an ozone-containing gas to decompose the ozone. More particularly, it is an object to provide a method that utilizes a high efficiency catalyst which operates at room temperature and does not have sites subject to poisoning. Other objects of the invention appear hereinafter.